1. Field of the Invention
The present invention is generally related to nuclear fuel elements and in particular to nuclear fuel elements used in particle bed reactors.
2. General Background
Particle Bed Reactors (PBR's) are cooled by pressurized gas that flows through the fuel elements. Each fuel element in a PBR is formed from two porous cylinders of different sizes. The smaller cylinder is positioned inside and concentric with the larger cylinder to define an annular space therebetween. The smaller and larger cylinders are commonly referred to in the industry as the inner and outer frits respectively. The annular space between the cylinders serves as the fuel bed by retaining nuclear fuel between the two cylinders. The nuclear fuel in the annular space is typically in the form of spherical particles of nuclear fuel material that are approximately 0.5 millimeter (500 microns) in diameter with each particle being coated to prevent the release of fission gas products during reactor operations. Cooling of the fuel elements is accomplished by directing the primary gas coolant through the outer cylinder so that it flows radially inward through the fuel particle bed and through the inner cylinder into the interior of the inner cylinder and then axially out the interior of the inner cylinder to heat exchange devices for converting the heat in the primary coolant to useful energy. Nuclear reactors may use either an open cycle system where the power fluid or coolant is used only once and replaced with fresh fluid or a closed cycle system where the same power fluid or coolant is used repeatedly. In order for an open cycle system to also be used as a closed cycle system (bimodal operation), it has been necessary to provide heavy, high-temperature valving to switch between the systems. Also, in gas-cooled particle bed reactors, it has been necessary to continue flow of the main coolant in order to remove decay heat from the reactor. Nuclear reactors and cooling systems which applicants are aware of include the following.
U.S. Pat. No. 4,847,040 discloses a liner cooling system for a high temperature gas-cooled reactor where the liner cooling system is arranged on the inside of the prestressed concrete pressure vessel for removal of decay heat.
U.S. Pat. No. 4,243,487 discloses a gas-cooled high temperature nuclear reactor where the steam generator includes a heat exchanger at least a portion of which is located above the reactor core to insure a natural convection flow of primary coolant for removal of decay heat when the reactor is shut down and the blowers are not functioning.
U.S. Pat. No. 4,382,908 discloses an after-heat removal system for a gas-cooled nuclear reactor in which the core is formed by a pile of spherical nuclear fuel elements. An outlet for gas carrying the after heat from the pile is provided in the side reflector in spaced relationship from the gas inlet and the gas outlet and opens within the pile and leads to a cooler which is in turn connected to the gas inlet of the reactor.
U.S. Pat. No. 4,689,194 discloses a gas-cooled nuclear reactor in which the core is formed by a pile of spherical nuclear fuel elements with a plurality of decay heat exchangers located in the steel pressure vessel and connected on the cooling water side with an external recooling heat exchanger each in a geodetically higher location for heat removal by natural convection.
U.S. Pat. No. 4,788,031 discloses the use of a plurality of cooling pipes mounted on the concrete jacket side of a liner with water flowing through the pipes for removal of decay heat from the nuclear reactor.
U.S. Pat. No. 4,144,723 is of general interest regarding primary and secondary cooling systems but does not disclose decay heat removal from a particle bed reactor.
U.S. Pat. No. 4,795,607 discloses a high temperature gas-cooled nuclear reactor having cooling tubes on the exterior of the vessel.
U.S. Pat. No. 4,678,626 discloses a liquid-metal pool breeder reactor with a surrounding shell around the containment vessel and fins that extend from the shell into the air space between it and the containment vessel. The fins are heated by radiation from the containment vessel and convect the heat to circulating air.
U.S Pat. No. 4,554,129 discloses a gas-cooled nuclear reactor wherein decay heat is removed by cooling tubes welded to the exterior or concrete side of the reactor vessel by conduction through the vessel liner.
Although a variety of gas-cooled reactors are known, none provide for bimode operation where an open or closed cycle system may easily be converted to the other or for the removal of heat generated at low power such as decay heat to generate low levels of continuous power.